Conventional implementations of IQ modulators are shown in FIGS. 1 and 2. M1 and M2 are two identical frequency multipliers, also referred to as frequency mixers. I and Q represent two input signals to be quadrature modulated. A summing device S1 sums the outputs of the I and Q circuit branches to generate an output signal (OUT).
In the circuit of FIG. 1, LO is a frequency signal that is generated by a Local Oscillator. LO is applied to a -45.degree. phaseshifter P1 and to a +45.degree. phaseshifter P2, and is thus phaseshifted by a total of 90.degree.. The phaseshifted LO signal is mixed with Q-IN and with I-IN by mixers M1 and M2, respectively, resulting in, ideally, 90.degree. of phaseshift between I and Q frequency components at the output of S1.
In the circuit configuration of FIG. 2 a single -90.degree. phaseshifter (P1') is interposed between LO and M1, and thus performs the same function as P1 and P2 in FIG. 1.
Although relatively straightforward to implement, the circuits of FIGS. 1 and 2 exhibit several problems. For example, both of these circuits require two separate mixers (M1 and M2) and, thus, require two separate balancing transformers in order to provide isolation between the LO signal and the I and Q signals. This is disadvantageous in that balancing transformers add significant cost and bulk to the circuitry. For consumer products, especially mobile communications products, it is generally desirable to minimize cost and size whenever possible.
It is known to employ transistor/FET mixers that do not require balancing transformers. However, transistor/FET mixers require bias potentials and DC power to operate. For battery powered systems, such as mobile communication systems, it is generally desirable to minimize circuit operating power so as to minimize battery recharging operations.
A further disadvantage of the circuits of FIGS. 1 and 2 is that the LO power must be sufficient to drive the phaseshifter(s), in addition to the mixers M1 and M2.
A still further disadvantage is that it is difficult to impedance match the outputs of the phaseshifters to the inputs of the mixers, in that the mixer input impedance is non-linear. As a result, the degree of phaseshift becomes a function of drive level.
It is thus an object of the invention to provide an IQ modulator that overcomes the foregoing problems.
It is another object of the invention to provide a low power, low cost phaseshifter circuit configuration that requires few components and no inductive components.